Protective coatings for aluminum and aluminum alloys



United States Patent ABSTRACT on THE DISCLOSURE 'A chromate conversion bath composition which when applied to aluminum or an alloy thereof imparts a corrosion resistant coating. The bath is an aqueous SOIUUOII. of hexavalent chromium, a water soluble fluorine contamlng compound yielding free fluorine ions in solution, a water soluble tungstate as Well as a water soluble phosphate or arsenate.

This invention relates to a treatment for aluminum and aluminum containing alloys and has as a principal aim provision of .a corrosion-resistant coating of good appearance which has improved wet hardness and non-smearing characteristics. In addition, the formation of the coating is substantially accelerated. Another principal aim of this invention is the provision of coating solutions of greatly improved stability.

It has been proposed by prior art that chromate conversion coatings can be produced on aluminum and alloys thereof by treatment with chemical baths of the following general compositions:

. (1) chromic acid (CrO salts thereof and mixtures 'ofthe same plus simple or complex fluorides or mixtures of the same. v g (2)Chromic acid, salts thereof and mixtures of the same with simple or complex fluorides or mixtures thereof=and ferricyanic or ferrocyanic acidsand mixtures of the same or their salts.

gThe chromate conversion coatings provided "by the baths disclosed above have left something to be desired in certain properties including uniformity, density and ,wet hardness.

It has also been proposed that improved chromate con version coatings can be obtained by using baths which include' tungstic acid and/or water solublesalts of tungstic acid, notably alkali metal salts in conjunction with one or more cations from the group-consisting of beryllium, magnesium, calcium, strontium and barium; the pH of the bath-being maintained within the range of about 1.3 to 2.2.

In brief, very careful testing and comparison of the chemical treating baths including tungstates disclose that, in general, the coatings form much more rapidly on the aluminum or aluminum alloy surface, are more uniform over the'sur'face ofthe treated metal, i.e., show less tendency toward iridescence or light spots or streaks, are denser and smoother and have markedly more pleasing appearance.

Coatings as formed in the treating bath are built up to a greater thickness without sloughing-off or becoming powdery or non-adherent. It has been observed that the coatings while still wet'have a much greater reance, to wiping or smearing than has heretofore been encountered. However, serious limitations are encountered with regard to stability and workability of treating solutions as described above.

First, the use of tungstic acid as such is virtually in- 3,410,707 Patented Nov. 12, 1968 ice effective since this compound is almost completely insoluble in acidic aqueous solutions.

Second, when water-soluble tungstates are used, tungstic acid tends to precipitate from solution on standing for a period of several days to several weeks, so thatth e concentration of tungstate ion remaining in solution is nil or so low as to be ineffective.

Third, the tungstate salts of the cations referred to above tend to be of limited solubility, especially the tungstates of barium and magnesium. Therefore, when these cations are used in conjunction with soluble tungstates, they cause the precipitation of tungstate from solution, so that the tungstate ion concentration remaining in solutionis essentially nil or sufliciently low as to reduce its effectiveness drastically. g

It is therefore a principal object of thepresent invention to provide a corrosion-resistant, uniform,'dense and smooth chromate conversion coating having improved wet hardness characteristics.

It is another object of the present invention to provide a coating bath of improved stability.

Still further objects and the entire scope of applicability of the present invention Will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by Way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

It has been found that stability of coating solutions containing water-soluble tungstate salts can be greatly im proved by the incorporation of phosphate or arsenate compounds along with the tungstate in the formulation. The additions of phosphates calculated at P 0 or arsenates calculated as As O in ratios of 1:10 to 1:150 to the tungstate calculated as W0 overcome a previously observed phenomenon of gradual precipitation of tungstic acid from solution on standing for a period of several days or weeks. Ratios over 1: 10 hinder formation of the conversion coating on aluminum, while ratios below 1:150 tend to be ineffective in the prevention of tungstic acid precipitation. The phosphate may be added as phosphoric acid or water-soluble salts thereof while the arsenate may be added as arsenic pentoxide or a water-soluble salt of arsenic acid.

It has been further found that the beneficial effects of tungstate ion, as recited above, are obtained from these solutions without the need of incorporating therein any of the cations previously referred to, namely, beryllium, magnesium, calcium, strontium, and barium. It is, in fact, for reasons given above, felt to be undesirable to incorporate any of these cations in the solution.

As examples of coating solutions containing soluble tungstate salts whose stability may be improved by the incorporation of phosphate or arsenate compounds are those containing also a chromic compound yielding a chromic ion (CH and a fluo compound yielding a fluoride ion (F*) in solution. The chromic ion may be provided by chromic acid or water-soluble salts thereof and the fluoride ion may be provided by simple fluorides such as alkali metal fluorides and bifluorides, or complex fluorides such as fluosilicates and fluoborates. Preferred compounds for providing the fluoride ion include fluosilicic acid and salts thereof and particularly certain alkali metal salts thereof. These complex fluorides are of limited solubility, thereby providing a simple means for carefully controlling fluoride ion concentration of a coating bath. However, such limited-solubility compounds are not essential to the proper functioning of the solution. Simple fluorides also give completely satisfactory results.

The above described coating solutions may also contain ferricyanic or ferrocyanic acid, water-soluble salts thereof and mixtures of the same.

The chromic acid and salts thereof are utilized in a range of concentrations equivalent to from 0.5 to 50 grams of CrO per liter of solution. The fluo compound may be employed in amounts equivalent to from 0.05 to 5 grams hydrofluoric acid, from 0.075 to 8 grams fiuosilicic acid and from 3.3 to 33 grams fiuoboric acid per liter of solution. The concentration of ferricyanic or ferrocyanic acid or salts thereof can vary from that equivalent to 0.1 to 50 grams of potassium ferricyanide per liter of solution. The soluble tungstate may be present in amounts equivalent to 0.035 to 7 grams of W per liter of solution. The amounts of phosphates or arsenates used depend on the tungstate concentration and the ratios, as set forth heretofore, are from 1:10 to 1:150 of phosphates calculated as P 0 or arsenates calculated as AS205 to the tungstate calculated as W0 In using any of the above coating baths, objects of aluminum and alloys thereof are immersed therein under the operating conditions discussed hereinafter.

In the following specific examples and throughout the specification and claims, all parts are parts by Weight unless otherwise specified.

In each of the foregoing examples an aqueous solution was employed and the panels were immersed in the same by dipping, However, the aqueous bath or solution could be sprayed or brushed or otherwise applied to the alumi num containing article to be protected. When necessary during coating operations, additions of about 1.0 ml./l. of nitric acid are used to offset rising of the pH of the treating solution.

It is to be understood that while it is preferred in all of the examples described in this case to use sodium tungstate because of its commercial availability, other soluble salts may be utilized, notably potassium tungstate, and, in fact, any of the water-soluble salts of tungstic acid.

In general, solution temperature is not critical. Temperatures within the range of 60 to 120 F. are found to be satisfactory but normal room temperatures, generally in the range of 70 to 90 F., are preferred for convenience.

Treatment time may vary from as low as 5 seconds to as high as minutes, depending on concentration and pH of the solution, and thickness of coating desired. For most immersion applications a treatment time between 1 and 6 minutes is preferred for convenience.

EXAMPLE 1 A coating solution was prepared having the following composition:

Ingredient: Amount in g./l. CIO 5 NaHF 0.5 NSQWOQ K PO 0.03 pH 1.6

A test panel of 2024-T3 was immersed for two minutes at at temperature of 75 F. The solution reacted faster than without Na WO and K PO present and produced a uniform golden brown coating, having superior resistance to wiping while wet.

EXAMPLE 2 Ingredient: Amount in g./l. CF03 5 NaHF2 0.5 K Fe CN 6 1 N32WO4 1 K PO 0.03 pH 1.6

A test panel of 2024T3 was treated as in Example 1.

Cit

A uniform medium brown coating was formed having superior resistance to wiping while wet.

EXAMPLE 3 The procedure of Example 2 was repeated using 0.02 g./l. of Na HPO in place of the K PO with comparable results.

EXAMPLE 4 Y Ingredient: Amount in g./l. CrO 7.49 NaB F 3 .00 Na WO 2.25 K PO 0.075 pH 1.4

A six minute treatment at F. of a 2024-T3 test panel resulted in a uniform light golden brown coating of superior wet hardness (resistance to wiping while wet).

A 'six minute treatment at 80 F. of a 2024-T3 test panel produced a uniform medium brown coating of superior wet hardness. The treated panel showed no corrosion after 500 hours exposure to 5% salt spray.

EXAMPLE 6 Ingredient: Amount in g ./l. CrO 5 .0 NHZSIFG 8 Na WO 0.5 K PO 0.03 pH 1.6

A uniform golden yellow coating of superior wet hardness' was produced on a 2024-T3 test panel immersed for three minutes in the above solution at 80 F.

EXAMPLE 7 Ingredient: Amount in g./l.- CF03 5 .0 Na SiF 8.4 K Fe(CN) 5.0 Na WO 0.5 K3PO4 pH 1.6

A 2024-T3 test panel treated as in Example 6 resulted in the formation of a uniform golden brown coating of superior wet hardness.

EXAMPLE 8 The procedure of Example 6 was repeated using 0.09 gram per liter of AS205 in place of the K PO with comparable results.

EXAMPLE 9 The procedure of Example 7 was repeated using 0.09 gram per liter of AS205 in place of the K PO with comparable results.

The solutions of the foregoing examples may be prepared from suitable dry powdered mixtures of the respective ingredients. It may be found advantageous, from the standpoint of stability of the powdered mixture, to prepare this in two separate parts.

A typical 2-part powdered mixture may be made up as follows:.

Mixture A Ingredients: Percent by weight CrO 87:7 Na HPO anhydrous 0.8 Na SiF 11.5

Mixture; B

Ingredients I N zs Es;

A preferred working mixture is prepared as follows:

When test panelsa of 3003, 20Z41T3 and 6061-T6 aluminum alloys were imersed the above solution at 80 F., uniform goldenhrown coatings were formed having superior wet hardness. The color ofthe coating was similar for all three alloys: Thecoat'ed 2024-T3 panels showed no corrosion after 500.hours exposure to salt spray.

It has also been found advantageous to prepare the coating bath from a 2 part liquid concentrate instead of from a dry powder mixture. The principal advantage of the liquid concentrates over the dry powder mixture is, in addition to greater stability of the chemical components in storagetogether, convenience in handling and preparation of working bath. For example, there is immediate complete solubility in preparation of the bath' and a minimum formation of insolubles or sludge during use thereof.

A typical 2 part liquid concentrate may be made up as shown in the following table:

Concentrate A Ingredient: Amount Chromic acid, CrO lbs 1.5 Phosphoric acid, H PO 85% ml 3.2 Nitric acid, HNO 40 Baum do 76 Fluosilicic acid, H SiF 30% do 356 Water to make 1 gallon.

Concentrate B Ingredient: Amount Sodium tungstate, Na WO 2H O lb 0.75 Potassium ferricyanide, K Fe(CN) do 0.60

Water to make 1 gallon.

Working solutions may be prepared by diluting concentrates A and B within the ranges of from /2 volume percent of each to 10 volume percent of each. For most purposes, ranges of from 1% to 5 volume percent of each concentrate are suitable.

A typical working solution may be prepared as follows:

EXAMPLE 11 Ingredient: Volume percent Concentrate A 2 /2 Concentrate B 2 /2 Water to make remainder.

or ferrocyanides. The amounts of hexavalent chromium expressed as CrO as well as the amount of the flue acids or salts thereof, of phosphoric acid or salts thereof, and

of fern'cyanides or ferrocyanides, where used, are in ac- .cordance with the ranges set forth heretofore.

" 1.;Arcompositi rm for use in aqueous solution .to im' ,part a corrosion resi stant coating to aluminum and alloys thereof in which the coatingr'producing' ingredients comprise. a hexavalent chromium compound selected from the. group consisting of chromic acidand water-soluble "salts thereof present, in amounts 'of 0.5 to'50 grams calculated as CrQ per liter of .said solution; a water-soluble fluorine containing compound yielding free fluoride ions in said solution present in amounts equivalent to. 0.05 to 5 grams of hydrofluoric acid per-liter of solutionfa watersoluble tungstate present in amounts'of 0.05m 10 grams calculatedas WO per liter of solution; and a phosphate selected from the'group consisting of phosphoricacid and water-soluble salts thereof calculated as P 0 and present in a ratio. of from 1:10 to l to said tungstate calculated asWO Y 2. The composition of claim 1 wherein the watersoluble fluorine containing compound is selected from the group consisting of hydrofluoric acid, fluoboric acid, fluosilicic acid, the water-soluble salts of said acids and mixtures of any of these acids and salts.

3. The composition of claim 1 including a cyanic acid selected from the group consisting of ferricyanic acid, ferrocyanic acid, water-soluble salts thereof and mixtures of said acids and salts present in amounts of 0.1 to 50 grams per liter calculated as potassium ferricyanide.

4. An aqueous solution to impart a corrosion-resistant coating to aluminum and alloys thereof in which the ingredients comprise water, a hexavalent chromium compound selected from the group consisting of chromic acid and water-soluble salts thereof present in amounts of 0.5 to 50 grams calculated as CrO per liter of said solution; a water-soluble fluorine containing compound yielding free fluoride ions in said solution present in amounts equivalent to 0.05 to 5 grams of hydrofluoric acid per liter of solution; a water-soluble tungstate present in amounts of 0.05 to 10 grams calculated as W0 per liter of solution; and a phosphate selected from the group consisting of phosphoric acid and water-soluble salts thereof calculated as P 0 present in a ratio of from 1:10 to 1:150 to said tungstate calculated as W0 5. The aqueous solution of claim 4 wherein the watersoluble fluorine containing compound is selected from the group consisting of hydrofluoric acid, fluoboric :acid, fiuosilicic acid, the water-soluble salts of said acids and mixtures of any of these acids and salts.

6. The composition of claim 4 including a cyanic acid selected from the group consisting of ferri-cyanic acid, ferrocyanic acid, water-soluble salts thereof and mixtures of said acids and salts present in amounts of 0.1 to 50 grams per liter calculated as potassium ferricyanide.

7. The composition of claim 4 wherein the watersoluble fluorine compound is present in amounts of 0.05 to 5 grains of hydrofluoric acid, 0.075 to 8 grams of fluosillcic acid and 3.3 to 33 grams of fluoboric acid and wherein said tungstate is present in amounts of 0.05 to 10 grams calculated as W0 per liter of solution.

8. A method of imparting a corrosion-resistant coating to aluminum and alloys thereof which comprises subjecting the latter to an aqueous acidic solution comprising water, a hexavalent chromium compound selected from the group consisting of chromic acid and water-soluble salts thereof present in amounts of 0.5 to 50 grams calculated as CrO per liter of said solution, a water-soluble fluorine containing compound yielding free fluoride ions in said solution present in amounts equivalent to 0.05 to 5 grams of hydrofluoric acid per liter of solution, a watersoluble tungstate present in amounts of 0.05 to 10 grams calculated as W0 per liter of solution and a phosphate selected from the group consisting of phosphoric acid and water-soluble salts thereof calculated as P 0 present in a ratio of from 1:10 to 1:150 to sand tungstate calculated as W0 9. The method of claim 8 wherein the water soluble fluorine containing compound is selected from the group consisting of hydrofluoric acid, fluoboric acid, fluosilicic acid, the water-soluble salts of said acids and mixtures of any of these acids and salts.

10. The method of claim 8 including a cyanic acid selected from the group consisting of ferricyanic acid, ferrocyanic acid, water-soluble salts thereof and mixtures of said acids and salts present in amounts of 0.1 to 50 grams per liter calculated as potassium ferricyanide.

11. A method according to claim 8 wherein the temperature of the solution is between about 60 and 120 F.

12. A method according to claim 8 wherein the pH of the solution is between about 0.5 and 3.0 and wherein the treatment time is between about 5 seconds and 15 minutes.

13. The method of claim 8 wherein the water-soluble fluorine compound is present in amounts of 0.05 to 5 grams of hydrofluoric acid, 0.075 to 8 grams fiuosilicic acid and 3.3 to 33 grams of fluoboric acid and wherein said tungstate is present in amounts of 0.05 to 10 grams calculated as W0 per liter of solution.

14. A method of imparting stability to an aqueous solution containing a water-soluble tungstate salt which comprises adding to said solution a phosphate selected from the group consisting of phosphoric acid and watersoluble salts thereof calculated as P 0 and present in a ratio of from 1:10 to 1:150 to said tungstate calculated as W0 References Cited UNITED STATES PATENTS 2,796,371 6/1957 Ostrander et a1 1486.2 2,868,679 1/ 1959 Pimbley 148-6.27 X 2,936,254 5/1960 Newhard et a1. 1486.27

FOREIGN PATENTS 513,376 5/1955 Canada.

JULIUS FROME, Primary Examiner.

L. B. HAYES, Assistant Examiner. 

